The size of wireless terminals has been decreasing with many contemporary wireless terminals being less than 11 centimeters in length. Correspondingly, there is increasing interest in small antennas that can be utilized as internally mounted antennas for wireless terminals. Inverted-F antennas, for example, may be well suited for use within the confines of wireless terminals, particularly wireless terminals undergoing miniaturization. Typically, conventional inverted-F antennas include a conductive element that is maintained in a spaced apart relationship with a ground plane. Exemplary inverted-F antennas are described in U.S. Pat. Nos. 5,684,492 and 5,434,579, which are incorporated herein by reference in their entirety.
Furthermore, it may be desirable for a wireless terminal to operate within multiple frequency bands in order to utilize more than one communications system. For example, Global System for Mobile communication (GSM) is a digital mobile telephone system that typically operates at a low frequency band, such as between 880 MHz and 960 MHz. Digital Communications System (DCS) is a digital mobile telephone system that typically operates at high frequency bands, such as between 1710 MHz and 1880 MHz. In addition, global positioning systems (GPS) or Bluetooth systems use frequencies of 1.575 or 2.4-2.48 GHz. The frequency bands allocated for mobile terminals in North America include 824-894 MHz for Advanced Mobile Phone Service (AMPS) and 1850-1990 MHz for Personal Communication Services (PCS). Other frequency bands are used in other jurisdictions. Accordingly, internal antennas are being provided for operation within multiple frequency bands.
FIG. 1 illustrates one example of a prior art PIFA (planar inverted “F” antenna) that uses a center signal fed planar antenna shape with capacitive coupling 10. Generally stated, the high band element has an end portion that typically capacitively couples to a closely spaced apart end portion of the low band element, which, in operation, may cause a larger portion of the antenna element to radiate. U.S. Pat. No. 6,229,487 describes similar configurations for wireless devices, the contents of which are hereby incorporated by reference as if recited in full herein. Unfortunately, the increase in the coupling between the two elements by this configuration may result in degradation in bandwidth at the low-band element. In addition, the parasitic element may dictate tight manufacturing tolerances for proper operation that may increase production costs.
Kin-Lu Wong, in Planar Antennas for Wireless Communications, Ch. 1, p. 4, (Wiley, Jan. 2003), illustrates some potential radiating top patches for dual-frequency PIFAS. As shown, the PIFA in FIG. 1.2(g) has a plurality of bends, but the configuration is such that the capacitive coupling between the two branches (primary and secondary branches) is most likely very large.
Despite the foregoing, there remains a need for alternative multi-band planar antennas.